This paper presents a conclusion to the meaning of the historical diagnosis "general paralysis of the insane" in nineteenth century Victorian Registrar General's reports. Most studies suggest that in the past the diagnosis refers to neuro-syphilis, and while after 1906 this is not disputed, this paper will show that the diagnosis had a much broader meaning incorporating many forms of degenerative brain disease during the study period (Victoria 1886 to 1906). It is acknowledged, however, that the use of this diagnosis may have varied dramatically in the past based on a doctor's education and/or background. This conclusion has been arrived at by analysing both government and general hospital records on the Australian, Victorian population. This paper also presents methods for determining the meaning of obsolete medical diagnoses where the definition is poorly understood.